Recently a brush-less DC motor has been increasingly employed in compressors used in refrigerating systems because of its high efficiency. A conventional compressor is known to work in the following manner: Detect a rotor position using back electromotive force (BEMF) yielded in stator windings of a motor, and drive the motor based on the detection signal, at the same time, chop the switching elements, thereby practicing the pulse width modulation control. An instance of such a conventional compressor is disclosed in Japanese Patent Application Non-Examined Publication No. H03-55478.
A controller of the conventional compressor is described hereinafter with reference to FIG. 8-FIG. 10. FIG. 8 shows a top view of a stator of a motor employed in the conventional compressor. FIG. 9 shows a top view of a rotor of the motor employed in the conventional compressor. FIG. 10 is a circuit diagram of the controller employed in the conventional compressor.
In FIG. 8, stator 1 of the motor is equipped with plural teeth 5 formed on core 3. Each one of teeth 5 is wound by concentrated windings 7. In FIG. 9, rotor 10 of the motor forms an interior permanent magnet (IPM) rotor in which four plate-like permanent magnets 14 are buried in iron core 12. In FIG. 10, motor 30 comprises stator 1 shown in FIG. 8 and rotor 10 shown in FIG. 9, and activates compressing mechanism 32 via a shaft (not shown). Rectifying circuit 36 for rectifying the AC of commercial power 34 adopts a voltage doubler rectifying method, so that it receives AC100V and outputs DC250V.
Inverter 40 is formed by bridging six pieces of switching elements for a three-phase operation. Inverter 40 converts the DC voltage output from rectifying circuit 36 into an output having a voltage and a frequency for three phases, thereby powering motor 30. Each one of the three phases is energized at 120 degrees in electric angles, so that an alternating current of rectangular waveform is supplied to motor 30.
Back electromotive force (BEMF) detecting circuit 42 detects a relative rotor position with respect to the stator by using BEMF yielded in the respective stator windings of the three phases of motor 30. Driver circuit 46 turns on or off the switching elements of inverter 40. Commutating circuit 48 determines which switching element of inverter 40 be turned on or off based on an output signal from BEMF detecting circuit 42 while motor 30 is in steady operation. PWM control circuit 50 chops switching elements either one of the upper arm or the lower arm of inverter 40, thereby carrying out PWM (pulse width modulation) control.
The PWM control refers to raising/lowering of an average output voltage by raising or lowering the duty of pulse width. The duty is defined in this specification as a ratio of an on-period vs. a pulse cycle.
An operation of the controller of the conventional compressor discussed above is described hereinafter. When motor 30 is activated from a stopped state, it is impossible to detect a rotor position because the rotor windings do not yield BEMF yet. Thus inverter 40 compulsorily outputs a voltage having a low frequency and a low duty. Application of the output voltage to the stator windings compulsorily starts the motor rotating. This is generally referred to as a sync. at a low frequency for energizing.
The motor thus starts rotating and increases its rpm to a certain level, then stator windings of respective phases yield BEMF, and BEMF detecting circuit 42 outputs a rotor position detecting signal. Commutating circuit 48 logically processes the position detecting signal, and outputs a commutating signal to drive circuit 46. Based on the commutating signal, drive circuit 46 turns on/off the six switching elements of inverter 40 one by one, thereby powering the respective phases of the stator windings one by one. The motor thus works steadily (under the feedback control by the position detecting signals) as a DC motor.
With respect to the DC motor, variation of a voltage applied to the motor can control rpm. Therefore, increment of duty in PWM based on a signal supplied from PWM control circuit 50 raises an average of the voltages applied to the motor, so that the motor increases its rpm. On the contrary, decrement of the duty lowers the average of the voltages applied to the motor, so that the motor reduces its rpm.
Since the position detecting signal supplied from BEMF detecting circuit 42 synchronizes with the rotation of the rotor, the rpm can be detected by this signal. The detected rpm signal is compared with a speed reference signal, and the comparison result is fed back for adjusting the duty, thereby controlling the rpm of the motor.
Meanwhile, the duty is defined by the following equation:Duty={on period/(on period+off period)}×100. For instance, when an on-period is 50% and an off-period is 50%, the duty becomes 50%.
The foregoing conventional structure controls the rpm using a pulse duty supplied from PWM control circuit 50, and a chopping frequency (hereinafter referred to as a carrier frequency) in PWM ranges from several kHz to ten and several kHz in general, so that the carrier frequency is accompanied with noises.
Since IPM rotor includes permanent magnets 14 therein, a magnetic path coupling iron core 12 of rotor 10 to teeth 5 of stator 1 is formed. Therefore, when a current having a rectangular waveform is supplied to stator windings 7, the magnetic path is switched to the adjacent tooth 5 at switching of powering a phase, so that magnetic force sharply changes. As a result, stator 1 is deformed, which results in generating noises.
Since the rpm is controlled by a pulse duty, the max. output is achieved at a duty of 100%, so that the motor cannot work at a higher rpm than the rpm at this level. In order to obtain a necessary capacity of the compressor, an output of motor 30 must be increased, and in the case of using the same amount of copper as the stator windings, the motor efficiency lowers by an increased amount of output.
In order to solve the problems discussed above, three-phase sine-waveform AC instead of a rectangular waveform is applied to stator windings 7 of motor 30 so that noises can be reduced. However, a method of applying the three-phase sine-waveform AC needs a current detecting circuit for detecting a current flowing through the stator windings in order to calculate a position of the rotor, because it is difficult to obtain information about detecting a position of the rotor from the BEMF detecting circuit. In such a case, a current transformer is used in general for detecting the current, and a high-speed microprocessor is required for calculating the rotor position. As a result, the method of applying the three-phase sine-waveform AC becomes expensive.